The present invention relates to a cold-cathode type electron-emitting element that brings about the field emission of electrons and to an image display device constructed using the same.
In recent years, accompanying increasing demands for a reduction in the thickness of displays and improved compactness, the development of microminiature electron-emitting elements capable of high-speed operation has become very active.
In the development of electron-emitting elements, research and development was first focused on the thermal emission type, but in recent years, research and development of the cold cathode type, which without requiring heating at high temperatures, can emit electrons even at a low voltage, has become extensive. Against this background and building on prior art, a cold-cathode type element structure capable of obtaining a stable, high current at low voltage and low power consumption and bringing about the emission of electrons from a selected location was proposed in the Japanese Unexamined Patent Publication No. H10-199398.
This element, as is shown in FIG. 8(a), takes on a structure such that a graphite layer 212, serving as the cathode electrode, is deposited in a line formation on a substrate 211, and on top of this, an electron-emitting layer 213 comprising a carbon nanotube layer is provided. In addition, an insulating region 214 is provided on both sides of the electron-emitting layer 213, and on top of this, a grid electrode 215 with a line formation is disposed so that it is orthogonal to the electron-emitting layer 213.
With this kind of structure, when a positive voltage is applied to the grid electrode 215 and a negative voltage to the cathode electrode 212, an electric field is generated at the sections where the electrodes intersect, and electrons are pulled from the intersecting sections of the cathode electrode. Therefore, by selecting a line to which to apply a voltage, it is possible to bring about the emission of electrons from a selected location. In addition, because the electron-emitting layer is made of carbon nanotubes, which have excellent discharge characteristics, a stable, large current can be obtained in a low vacuum and at a low voltage.
However, with this element structure, there are the following inherent problems:
1) Because the restricting of the pulling of electrons is accomplished only by the potential difference between the cathode and the grid, in order to pull electrons from the cathode, sufficient voltage must be applied to the grid electrode. Thus, it is difficult to realize a sufficiently low operation voltage.
2) Because an electric field is generated between the opposing surfaces of the intersecting cathode electrode and the grid electrode, many electrons emitted from the cathode electrode surface end up flowing into the grid electrode, which is the opposing surface. Therefore, the number of electrons that arrive at the anode, which is disposed above the grid electrode, is not more than the small portion that passes through the central portion of the electron passage hole. Thus, the usage efficiency of the emitted electrons is low.
3) The anode electrode is disposed above the grid electrode, but when a potential is supplied to the anode electrode, an electric field concentration arises at the edge portions of the grid electrode, and thus the generating of abnormal discharge from the edge portions of the grid 215 tends to occur. Abnormal discharge causes a considerable deterioration in the reliability of the electron-emitting device.
The present invention was developed to solve the foregoing and other problems. The inventors of the present invention, through intense research, discovered that electrons could be very efficiently pulled from an electron-emitting material (the cathode electrode) by combining an electric field between the anode electrode and the cathode electrode and an electric field between the grid electrode and the cathode electrode and that abnormal discharge from the edge portions of the grid electrode could be prevented by adjusting the placement and shape of the grid electrode. The inventors thus achieved the present invention. The present invention has the following construction.
(1) An electron-emitting element is provided comprising an electron conveying member, a cathode electrode comprising an electron-emitting member fixed to the electron conveying member, an anode electrode disposed such that it is spaced from the cathode electrode, and a grid electrode disposed between the cathode electrode and the anode electrode and having an electron passage opening, wherein the spatial positioning of the three members, the cathode electrode, the anode electrode and the grid electrode, and their respective shapes are constructed such that an electric field existing between the grid electrode and the anode electrode emanates from the electron passage opening to the cathode electrode side, and the emanated electric field and an electric field existing between the cathode electrode and the grid electrode interact with each other to form a combined electric field, and electron-emission controlling means is provided for varying the intensity of the combined electric field by varying the potential of at least one of the cathode electrode, the anode electrode, and the grid electrode to control the number of electrons emitted from the cathode electrode.
This construction is characterized in that a combined electric field is formed from the electric field that emanated to the cathode electrode side and the electric field generated between the cathode electrode and the grid electrode and the field emission of electrons from the cathode electrode is controlled by varying the intensity of the combined electric field. Compared with conventional field emission elements, an element employing this control method makes remarkably efficient field emission possible. Thus, at a low operating voltage, there is good response and stable electron emission can be brought about. The basic principle of this kind of electron-emitting element of the present invention is described with reference to FIG. 1.
In an electron-emitting element of the present invention, in order to form a combined electric field, the spatial positioning and shapes of the three members, the cathode electrode, the anode electrode, and the grid electric, are appropriately adjusted, and by utilizing the electron-emission controlling means, the intensity of the combined electric field is controlled to control the number of electrons emitted from the cathode electrode. The characteristics and technical significance of this combined electric field is made clear by FIG. 1.
FIG. 1 shows the concept that supposing a voltage much lower than but with the same polarity as that of the anode electrode is applied to the grid electrode, the state of the combined electric field is as represented by equipotential surfaces 10. As is shown in FIG. 1, an electric field existing between a grid electrode 3 and the anode electrode emanates from the electron passage opening of the grid electrode 3 to the cathode electrode side and this emanated electric field interacts with an electric field generated between a cathode electrode 2 and the grid electrode 3 to form a protruding set of equipotential surfaces on the cathode electrode side. This set of equipotential surfaces is the combined electric field.
A combined electric field region 11 of FIG. 1 designates the range of the effect of the emanating electric field, in other works the range of the combined electric field. The respective intervals of the group of equipotential surfaces within the combined electric field region 11 are, as shown in FIG. 1, the most narrow on the line (valley line) connecting the points (valleys) of each equipotential surface, and as the distance from the valley line increases to either the right or the left, the equipotential surface intervals widen. In other words, the group of points on the valley line have the largest potential differences, and as the distance from the valley line increases to either the right or the left, the potential differences decrease. In addition, this valley line is orthogonal to an anode electrode surface and the cathode electrode. This aspect of the combined electric field gives rise to the following positive effects at the time of electron emission.
First, because the valley line shows potential differences larger than other regions, in the valley line portion where the valley line intersects with the cathode electrode, the effect on the pulling of electrons from the cathode electrode increases. The pulled electrons are then guided along the valley line, which shows the largest potential differences, and arrive at the anode electrode. Thus, there is little decline in electron usage efficiency due to the absorbing of electrons by the grid electrode. In other words, according to the method of the present invention, which employs a combined electric field, the valley line consistently guides electrons emitted from the cathode electrode to the anode electrode, having the function of a tunnel for flying electrons as it were. Thus, by even lower grid electrode voltages, it is possible to make electrons pulled from the cathode electrode efficiently arrive at the anode electrode.
It was confirmed through experiment that when a combined electric field is employed, the effects and the like on the pulling of electrons from the cathode electrode are remarkably increased, but the relationship between the individual electric fields and the combined electric field, the state of the distribution of electric potential within the combined electric field, and the like are not at the present fully understood. It is to be noted that potential differences in conventional elements that do not employ a combined electric field are such that equipotential surface intervals are as shown on the outside of the combined electric field region 11 in FIG. 1.
As described above, according to the above-described construction of the present invention, the potential applied to the anode electrode and the grid electrode can be very effectively utilized for the pulling of electrons from the cathode electrode, and thus compared with field emission type elements having conventional constructions, an even greater number of electrons can be consistently pulled from the cathode electrode with even less power.
(2) The present invention may further take on the following construction.
The work function of at least an anode electrode side surface of the grid electrode may be larger than the work function of the cathode electrode. This construction makes it possible to prevent the abnormal discharge of electrons from a surface of the grid electrode in the direction of the anode electrode.
The grid electrode may be earthed by means of an electric circuit through which electrons do not flow from the earthed side. This construction makes it possible to prevent abnormal discharge from the grid electrode.
The grid electrode may be disposed between the cathode electrode and the anode electrode so that at least the relationship dxe2x89xa7Z1 is satisfied, where d is the maximum opening length of the electron passage opening and Z1 is the vertical distance from a cathode electrode surface to a surface on the cathode electrode side of the grid electrode. This construction makes it possible for the electric field between the anode electrode and the grid electrode to easily spread from the electron passage opening of the grid electrode.
An electric field concentration reducing means for reducing an electric field concentration from the anode electrode may be performed in the vicinity of the electron passage opening of the grid electrode, and the electric field concentration reducing means may be such that the work function of a perimeter edge portion on the anode electrode side of the electron passage opening of the grid electrode is larger than the work function of other portions of the grid electrode. In addition, it may be such that at least the perimeter edge portion on the anode electrode side of the electron passage opening of the grid electrode is chamfered. By employing this construction, abnormal discharge from the vicinity of the electron passage opening is prevented.
The electron-emission controlling means in the construction of the present invention may be such that the potential of the anode electrode with respect to the cathode electrode is constant and the strength of the combined electric field is varied by varying the potential of the grid electrode. The present invention employs a construction such that the electric field on the anode electrode side is emanated from the opening of the grid electrode to the cathode electrode side and is combined with the electric field generated between the grid electrode and the cathode electrode. This construction makes it possible to vary the characteristics and intensity of a combined electric field by varying only the potential of the grid electrode, thus making it possible to vary the number of electrons emitted from the cathode electrode. As a result, by appropriately adjusting the potential of the anode electrode, the grid electrode potential, serving as the potential for controlling the number of electrons emitted from the cathode electrode, can be made very small.
The electron-emission controlling means may be such that the potential of the anode electrode with respect to the cathode electrode is a potential at which the field emission of electrons from the cathode electrode in the direction of the anode electrode cannot be brought about by only the potential thereof, the potential of the grid electrode has the same polarity as that of the anode electrode, and by varying the potential of the grid electrode, the strength of the combined electric field is varied.
Even if the potential of the anode electrode with respect to the cathode electrode is a potential at which the field emission of electrons from the cathode electrode in the direction of the anode electrode cannot be brought about by only the potential thereof, by appropriately arranging and so forth the spatial positioning and the respective shapes of the three members, the cathode electrode, the anode electrode, and the grid electrode, an electric field is emanated from the electron passage opening of the grid electrode and this emanating electrode acts on the electric field between the grid electrode and the anode electrode making it possible to form a combined electric field. The intensity and the like of this combined electric field can be varied by varying the grid electrode potential by utilizing the electron-emission controlling means. This makes it possible to easily control electron emission from the cathode electrode.
The electron-emission controlling means may be such that the potential of the anode electrode with respect to the cathode electrode is a potential at which the field emission of electrons from the cathode electrode in the direction of the anode electrode can be brought about by only the potential thereof, and by varying the potential of the grid electrode, the strength of the combined electric field is varied. With this construction, the emission of electrons from the cathode electrode can be controlled as in the above construction, but in this construction, the potential applied to the grid electrode may be varied within the range of plus to 0 to minus to control field emission. Specifically, when restricting the emitting of electrons from the cathode electrode, a potential having a reverse polarity to that of the anode electrode is applied to the grid electrode, and when a further increase in the number of electrons pulled from the cathode electrode is desired, a potential having the same polarity as that of the anode electrode is applied.
The cathode electrode of a construction of the present invention may comprise an electron-emitting member formed into a columnar shape, and the electron-emitting member may be disposed so that an extension line in the tip direction of the electron-emitting member passes through the electron passage opening and is orthogonal to an anode electrode surface. According to the present invention, as is shown in the FIG. 1, the valley line, which shows large potential differences, is formed perpendicularly to the anode electrode. Consequently, by making the electron-emitting member into a columnar shape and by disposing this member preferably so that it coincides with the valley line, an electric field concentration is generated in the tip portion of the columnar electron-emitting member, and thus electron emission from the member is simplified and because the equipotential surface density on the side surfaces of the column is high, efficient discharge is made possible. In other words, because the zone of large potential differences can be effectively utilized, this construction makes high current discharge at a low operating voltage possible. It is to be noted, based on the conceived form of the combined electric field in FIG. 1, that discharge efficiency improves when disposing a columnar electron-emitting member along the valley line within the combined electric field region more than when using a planar electron-emitting member.
The electron-emitting member may have a shape such that the relationship rxe2x89xa60.3D is satisfied, where r is the radius of curvature of a tip corner portion and D is the maximum width of the column. It is preferable that this shape be employed because, when employing this shape, an electric field concentration arises in the tip of the electron-emitting member.
The grid electrode and the cathode electrode may be constructed such that the relationship dxe2x89xa7Z1 is satisfied, where d is the maximum opening length of the electron passage opening of the grid electrode and Z1 is the vertical distance from a tip of the electron emitting member. It is preferable that dxe2x89xa7Z1 because the spreading of the anode potential gets larger when dxe2x89xa7Z1.
The grid electrode and the cathode electrode may be constructed such that the relationship Z1xe2x89xa60.25L is satisfied, where L is the height of the electron-emitting member from a surface of the electron conveying member and Z1 is the vertical distance from a tip of the electron-emitting member to a surface of the grid electrode. This condition makes it possible to further effectively utilize the spreading of the anode potential and to enhance the electric field concentration directed at the electron-emitting member.
The electron-emitting element may be constructed such that the electron-emitting member has a shape such that the relationship rxe2x89xa60.3D is satisfied, where r is the radius of curvature of a tip corner portion thereof and D is the maximum width of the column, the grid electrode is disposed such that the relationship Z1xe2x89xa60.25L is satisfied, where L is the height of the electron-emitting member from a surface of the electron conveying member and Z1 is the vertical distance from a tip portion of the columnar electron-emitting member to a surface of the grid electrode, and the size of the electron passage opening is fixed so that the relationship dxe2x89xa7Z1 is satisfied, where d is the maximum opening length of the electron passage opening of the grid electrode. This construction makes it possible, because the combined electric field can be very effectively utilized, to realize an electron-emitting element which can obtain a high current discharge at a low operating voltage.
The electron-emitting member may comprise a carbon-type material.
The electron-emitting member may comprise graphite having six-carbon rings with dangling "sgr" bonds. Graphite having six-carbon rings with dangling "sgr" bonds shows directivity in electron emission and thus is favorable as the electron-emitting member.
The electron-emitting member may comprise a crystal whisker substance. This is preferable because a crystal whisker substance is excellent for electron emission.
The electron-emitting member may comprise carbon fiber. Carbon fiber is preferable from the perspective of excellent discharge characteristics and a moderate price.
The electron-emitting member may comprise carbon nanotubes. Carbon nanotubes are preferable because the tips are rounded and it is excellent in terms of electron emission.
The cathode electrode may further comprise at least one other electron-emitting member fixed to a surface of the electron conveying member, the electron-emitting members having a columnar shape, the electron-emitting element may be constructed such that, between the electron-emitting members and the grid electrode, the relationships Pxe2x89xa70.5L and Z1xe2x89xa60.25L are satisfied, where P is the interval between each of the electron-emitting members and Z1 is the vertical distance from a tip of the electron-emitting member having the highest vertical height to a surface of the grid electrode. Using a cathode electrode having a plurality of electron-emitting members makes it possible to increase the number of electrons emitted more than is possible with a cathode having a single electron-emitting member under the same operating voltage. However, when the placement interval of the plurality of electron-emitting members is less than half of the length of the members, because the electric field concentration at each electron-emitting member weakens, the advantageous effects of providing a plurality of electron-emitting members is offset. For this reason, in order to sufficiently obtain the advantageous effects of providing a plurality of electron-emitting members, it is preferable to construct the cathode electrode such that Pxe2x89xa70.5L and Z1xe2x89xa60.25L.
In the case of a plurality of electron-emitting members also, the work function of at least an anode electrode side surface of the grid electrode may be larger than the work function of the cathode electrode. By making the work function of the anode electrode side of the grid electrode larger, prevention of abnormal discharge from the grid electrode is made possible.
In the case of a plurality of electron-emitting members also, the maximum length d of the electron passage opening of the grid electrode may be formed such that the relationship dxe2x89xa7Z1 is satisfied, where d is the maximum opening length of the electron passage opening of the grid electrode and Z1 is the vertical distance from the tip of the electron-emitting member having a vertical height L to the surface of the grid electrode.
In the case of a plurality of electron-emitting members also, electric field concentration reducing means for reducing an electric field concentration from the anode electrode may be performed in the electron passage opening of the grid electrode.
In the case of a plurality of electron-emitting members also, the electric field concentration reducing means may be such that the work function of the perimeter edge portion of the electron passage opening of the grid electrode on the anode electrode side is larger than the work function of other portions of the grid electrode.
In the case of a plurality of electron-emitting members also, the electric field concentration reducing means may be such that chamfering is performed on at least the perimeter edge portion of the electron passage opening of the grid electrode on the anode electrode side.
In the case of a plurality of electron-emitting members also, the grid electrode may be earthed by means of an electric circuit through which electrons do not flow from the earthed side.
In the case of a plurality of electron-emitting members also, the electron-emitting members may comprise a carbon-type material.
In the case of a plurality of electron-emitting members also, the electron-emitting members may comprise graphite having six-carbon rings with dangling "sgr" bonds.
In the case of a plurality of electron-emitting members also, the electron-emitting members may comprise a crystal whisker substance.
In the case of a plurality of electron-emitting members also, the electron-emitting members may comprise carbon fiber.
In the case of a plurality of electron-emitting members also, the electron-emitting members may comprise carbon nanotubes.
(3) An image display device of the present invention may have the following construction. An image display device comprises a plurality of electron-emitting elements, a circuit which is connected to each of the electron-emitting elements and which transmits electric signals to each of the electron-emitting elements for electron emission, and an image formation section for forming an image by means of electrons emitted from the electron-emitting elements, wherein the electron-emitting elements are any of the electron-emitting elements of the previously described aspects of the invention.
This construction, because it exhibits the advantageous effects of operation of any of the previously described electron-emitting elements, makes it possible to realize an image display device which can achieve highly accurate images at a low operating voltage.